1. Field of the Invention
This invention is directed toward an improved fast neutron detector, and more particularly directed toward the optimization of the detector efficiency when used in logging of earth formations penetrated by a borehole and for a variety of applications.
2. Background of the Art
In the context of this disclosure, xe2x80x9cloggingxe2x80x9d is defined as the measure of a parameter of material penetrated by a borehole, as a function of depth within the borehole.
There are many types or classes of borehole logging systems. These classes include, but are not limited to, electromagnetic, acoustic and nuclear systems. Each class of logging system typically comprises a xe2x80x9csourcexe2x80x9d which emits energy into the surrounding formation, and one or more xe2x80x9cdetectorsxe2x80x9d which measure energy returning from the formation. Detector responses, when properly analyzed and processed, yield formation and borehole parameters of interest.
Any type or class of logging system typically comprises a source and detector system with sufficient depth of investigation to penetrate the logging instrument housing, penetrate the immediate borehole region, enter the surrounding earth formation, interact with the formation, and induce some type of response which returns to the borehole and the logging instrument to be detected and analyzed. Nuclear logging systems typically involve the use and measure of gamma radiation and neutron radiation. These types of radiation have greater depths of penetration in comparison to protons, alpha particles and beta particles. As a result, nuclear logging instruments typically comprise a source of neutrons, or a source of gamma radiation, one or more neutron detectors, or one or more gamma ray detectors, or some combination of these different types of sources and detectors.
Logging instruments are typically conveyed along a borehole by means of a wireline or drill string thereby creating a xe2x80x9clogxe2x80x9d of formation parameters as a function of depth within the borehole. Borehole conditions are harsh in that temperatures and pressures are high. Components within a logging instrument, such as detectors, are subjected to these environmental conditions as well as vibration and impacts resulting from the conveying of the instrument along the borehole. As an example, nuclear detectors used in logging applications must be able to withstand these harsh conditions of the borehole environment including temperatures which can reach 175 degrees Centigrade (xc2x0 C.) or higher.
All nuclear logging systems involve the measure of statistical nuclear processes. As a result, statistical significance of the measurement is of prime importance since it directly affects the statistical precision of one or more parameters of interest computed from the measurement. Statistical precision improves as the number of detector events increases. It is therefore very desirable to maximize the efficiency of nuclear detectors used in borehole logging operations. Furthermore, space is often limited in downhole instrumentation making it of utmost importance to maximize detector efficiency for a given geometry allowed in the design of the instrument.
Attention will now be directed toward prior art neutron detectors. Liquid scintillators have been used to detect high energy or xe2x80x9cfastxe2x80x9d neutrons. These scintillators also respond to impinging gamma radiation. Neutron and gamma ray xe2x80x9ceventsxe2x80x9d generate different pulse shape responses from liquid scintillators. Pulse shape discrimination methods therefore provide means for separating fast neutron and gamma ray induced responses in liquid scintillator detectors. Fast neutron and gamma ray measurements can be made with a single liquid scintillator detector. Liquid scintillators are relatively efficient. Unfortunately, liquid scintillators consist of flammable mixtures, and some mixtures have very low flash points. For these reasons, liquid scintillators are not desirable for high temperature, high pressure downhole applications.
Gas filled detectors, such as detectors containing relatively high pressures of helium-4 (4He), have been used as fast neutron detectors. These detectors are relatively rugged, and can withstand relatively high temperatures encountered within the borehole. Because the detectors are gas filled rather than liquid or solid, their detection efficiency is relatively low, and therefore not particularly desirable for downhole applications where statistical significance of measured detector response is of prime importance.
Plastic scintillators are relatively efficient neutron detectors, rugged in construction, and able to operate at temperatures of at least 175xc2x0 C. These detectors are, however, responsive to both fast neutrons and gamma radiation. Neutron and gamma ray events can not be delineated by the shape or amplitude of pulses produced by the detector. The crystal anthracene, a hydrocarbon, is another type of solid material used in fast neutron detectors but, like the plastics, can not separate fast neutron from gamma ray events using pulse shape or pulse amplitude discrimination.
Stilbene and p-terphenyl crystals are fast neutron detectors and are reported to produce pulses which can be separated into fast neutron and gamma ray events. This class of detector does not have the flammability of the liquid scintillators. The crystals are, however, not rated as operable at temperatures of 175xc2x0 C. The crystals are also difficult to fabricate, and availability is questionable with the only known source being Russia.
A fast neutron detector potentially suitable for downhole applications is an activated zinc sulfide scintillator combined with a nonscintillating plastic. The activated dopant is preferably silver (Ag) but other elements, such as copper (Cu) may be suitable or even better activators depending on the application of the detector. Activated zinc sulfide will be denoted by the symbol xe2x80x9cZnSxe2x80x9d in the remainder of this disclosure, with the understanding that the dopant can consist of a variety of materials. The non scintillating plastic can be any hydrogen rich material that is optically transparent and that possesses suitable mechanical properties.
Geometrically, the detector is constructed with a ZnS cylindrical core surrounded by alternating and concentric cylinders of plastic and ZnS. The scintillator detector was first introduced by Emmerich in 1954 (W. S. Emmerich., Review of Scientific Instruments, vol. 25, page 69 (1954)). Neutron and gamma ray events can be separated by pulse amplitude discrimination. Fast neutron detectors of this type are offered commercially by the Bicron division of Saint-Gobain International Ceramics, Inc. The material in not flammable, and it is thought that the detector can meet a 175xc2x0 C. temperature rating with some modifications. The main disadvantage of this type of detector for borehole applications is the relatively small volume, with corresponding reduction in detector efficiency. Furthermore, efficiency is not maximized for specified detector volumes, and in particular for specified detector geometry of diameter D and length L. Detector volume is restricted by the lack of light transparency of ZnS, with scintillations within the ZnS element only being able to reach an optically coupled photomultiplier (PM) tube through the transparent plastic component of the detector. The plastic component of the detector contains hydrogen (H). As with other H containing fast neutron detectors, the material responds to fast neutrons impinging upon the detector by the proton recoil process, with recoil protons generating scintillations within the ZnS component of the detector. Detector response is further enhanced by a threshold (n,p) reaction with 32S as reported by Birks (J. B. Birks, The Theory and Practice of Scintillation Counting, Pergamon Press, page 548, Oxford, 1964). This reaction introduces additional neutron induced proton flux within the ZnS scintillation material thereby increasing the efficiency of the detector.
Measures of fast neutrons are used in many prior art well logging systems to determine formation and borehole parameters of interest. In these prior art systems, fast neutron fluxes are typically measured inefficiently, and in many cases are determined indirectly in that the other parameters are measured and used to compute fast neutron fluxes.
The prior art contains patents teaching various apparatus and method for measuring and applying neutron and gamma ray measurements to obtain parameters of earth formations penetrated by a borehole. Patents thought to be the most relevant to this disclosure are summarized as follows:
U.S. Pat. No. 4,122,339 to Harry D. Smith, Jr. et al discloses a logging system that irradiates, with fast neutrons, earth formations penetrated by a borehole. Fast neutron population is measured indirectly from inelastic scatter gamma radiation detected with a gamma ray detector during bursts of fast neutrons from a pulsed neutron source. An epithermal neutron detector is used to measure epithermal neutron population following each neutron burst. The inelastic scatter gamma ray measurement is then combined with a fast neutron/epithermal neutron ratio to determine formation porosity.
U.S. Pat. No. 4,122,340 to Harry D. Smith, Jr. et al discloses a logging system using epithermal and fast neutron detectors. A stilbene scintillation crystal is used to detect fast neutrons. Measurements of fast and epithermal neutrons are combined to determine formation porosity.
U.S. Pat. No. 4,134,011 to Harry D. Smith, Jr. et al discloses a logging system comprising one epithermal and one fast neutron detector. Formation porosity is determined by making a dual spaced fast to epithermal neutron measurement using a continuous source of fast neutrons. Stilbene is used in the fast neutron detector with a spacing from the neutron source of 40 to 80 centimeters (cm). Pulse shape discrimination is used to separate gamma ray events from fast neutron events.
U.S. Pat. No. 4,152,590 to Harry D. Smith, Jr. et al discloses a logging system which is very similar to the system disclosed in U.S. Pat. No. 4,134,011 summarized above. A thermal decay rate measurement is added.
U.S. Pat. No. 4,605,854 to Harry D. Smith, Jr. disclosed a logging system wherein earth formation is irradiated with fast neutrons. A single fast neutron detector is used to measure a resulting neutron energy spectrum by an unfolding process. The patent does not disclose specific detector type, and whether or not gamma ray discrimination is achieved.
U.S. Pat. No. 4,631,405 to Harry D. Smith, Jr. discloses a dual spaced fast/epithermal neutron porosity logging system. Fast neutrons are measured at a short spacing with respect to a fast neutron source, and epithermal neutrons are measured at a long spacing with respect to the neutron source. Measurements are combined to obtain formation porosity.
U.S. Pat. No. 5,068,532 to Malcolm R. Wormald et al discloses a system wherein fast neutrons are detected for the purpose of providing coincident-timing information in lieu of using a pulsed neutron source. The detector is not used to produce borehole logging information, although logging is mentioned in one application.
U.S. Pat. No. 5,008,067 to John B. Czirr discloses a method for monitoring the output of fast neutrons from a neutron source element of a well logging apparatus. The detector comprises a scintillator containing oxygen. The 16O(n,p)16N reaction induced by 14 MeV neutrons produces delayed and very large amplitude pulses resulting from the sum of detected beta-decay energy and the 6-7 MeV gamma radiation from the decay of 16N. These pulses can be separated from other neutron and gamma ray pulses.
U.S. Pat. No. 6,207,953, assigned to the assignee of the present application, discloses a logging system in which fast neutrons and inelastic scatter gamma rays are measured and combined to determine formation porosity (and therefore density), and also combined to determine formation liquid saturation. A liquid scintillator is identified for fast neutron detection, providing both fast neutron and inelastic gamma ray counts by pulse shape discrimination. An alternate plastic scintillator and gamma ray detector combination is also taught in the event that a liquid scintillator is not suitable for a particular application. Fast neutron energies are distinguished by use of pulse height discrimination to provide borehole size compensation for air filled boreholes.
In view of the above discussion of prior art, it is apparent that an improved detector for directly measuring fast neutron fluxes in harsh borehole environments is needed. Furthermore, it is apparent that a fast neutron detector with efficiency maximized for a given detector geometry is also needed. This disclosure addresses both of these needs.
A geometrically optimized fast neutron detector is fabricated of alternating regions of non scintillating, hydrogenous, optically transparent material and scintillation material. The interfaces between alternating regions are critical to the detector""s fast neutron response. One geometry comprises alternating, concentric, right cylinders of activated ZnS scintillator material and non scintillating plastic. The ZnS activator can be Ag or Cu or any other suitable activator. Again, the symbol ZnS is used to denote activated zinc sulfide, which can be activated with a variety of dopants. The detector, however, is not limited to cylindrical geometry and may utilize alternate types of scintillator material. The plastic denotes a material that is rich in hydrogen (H) and that is optically transparent. Fast neutrons interact with the plastic producing recoil protons which enter the ZnS scintillation material. The ZnS material is normally potted with a binder such as epoxy, which also contains H. Therefore, some proton recoils will also occur within the ZnS scintillator region. Protons create scintillations within the ZnS, and a portion of this light escapes the ZnS, enters the transparent plastic, and is detected by a photomultiplier (PM) tube which is optically coupled to the detector. The PM dynode string is electrically connected to pulse amplification circuitry. Recoil proton energy is a function of fast neutron energy impinging upon the plastic component of the detector. The intensity of the scintillation is a function the energy of recoil protons entering the ZnS scintillation material. The amplitude of the pulse from the amplification circuitry of the PM tube is a function of the intensity of the scintillation. The number of output pulses is a measure of fast neutron flux, and the amplitude of the pulses is a measure of fast neutron energy. Pulse amplitude is also affected by the position at which the proton recoil reaction occurs within the plastic material. This effect must be considered in using the detector in fast neutron spectrometry systems, as will be discussed in more detail in a
subsequent section of this disclosure. Recoil protons have a limited range within the plastic materials. Only proton recoil events occurring near a plastic-ZnS interface will enter the scintillation material and therefore create a scintillation. ZnS is not light transparent. As a result, only proton scintillation events occurring near a ZnS-plastic interface enter a transparent plastic cylindrical annuli, and are eventually detected by the PM tube and recorded as a fast neutron event.
There is also evidence that additional proton flux is generated within the ZnS scintillation material by fast neutrons through the 32S(n,p)32P reaction. These protons also create scintillations within the ZnS material.
For a given overall detector diameter D, efficiency can generally be increased by decreasing the radial wall thickness of the ZnS and plastic cylinders, thereby increasing the ZnS-plastic surface area. If, however, the wall thickness of the plastic cylinders is decreased too much, the cylinders cease to become an efficient source of recoil protons, and further cease to become a scintillation xe2x80x9clight pathxe2x80x9d to the PM photocathode. Furthermore, if the radial wall thickness of the ZnS cylinders is decreased too much, the cylinders will not scintillate all entering recoil protons. Stated another way, the ZnS and plastic cylinder wall thicknesses for maximum detector efficiency is a xe2x80x9ctrade-offxe2x80x9d, and these dimensions must be optimized for a given detector diameter D. Detector efficiency can also be increased by increasing the length L of the detector. Length is also a trade-off parameter in that excessive length can decrease the detector""s gamma ray rejection capability, and further decrease the efficiency of the plastic annuli as light paths to the PM photocathode.
The geometrically optimized ZnS/plastic fast neutron detector is ideally suited for use in any downhole instrument in which a measure of fast neutrons is desired. One application is disclosed in the previously referenced U.S. Pat. No. 6,207,953, assigned to the assignee of the present application, and hereby incorporated in this disclosure by reference. The logging system uses measures of fast neutrons and inelastic scatter gamma rays, which are combined to determine formation porosity (and therefore density), and also combined to determine formation liquid saturation. A pulsed neutron generator provides a source of fast neutrons. Sodium iodide is a suitable inelastic gamma ray detector, wherein the detector is wrapped with a thermal neutron absorbing material such as cadmium to prevent neutron activation of the crystal. A ZnS/plastic detector is used to measure fast neutrons, wherein the geometry of the detector is optimized for maximum efficiency for the space available for the detector within the instrument or logging xe2x80x9ctoolxe2x80x9d. A ratio of fast neutron energies is determined by use of pulse height discrimination to provide borehole size compensation for air filled boreholes. As mentioned previously, both impinging fast neutron energy and the position at which a neutron induced proton recoil event occurs within the plastic component of the detector affect measured pulse amplitude. It is necessary to account for proton energy loss, commonly referred to as xe2x80x9cdE/dxxe2x80x9d, as the proton moves from the plastic component into the scintillation component, as will be discussed subsequently.
The geometrically optimized detector is suited for use as a monitor of output from fast neutron sources. This application is not only applicable to well logging apparatus and methods, but also applicable to a wide range of analytical and testing methods and apparatus which use fast neutron sources.